The invention refers to a device for electrically measuring a level of a liquid contained within a container, having a conductive foil which is surrounded by a protective tube and extends into the liquid.
The foil is either a temperature-dependent, electrically externally heated resistance foil whose total resistance as determinable by an evaluation circuit is dependent on the level of the liquid in the container, or a capacitance foil bearing two conductors which form a capacitor. In both cases the conductive foil is formed as a flat band, and is held untwisted in the protective tube by holding members which act on at least one of the longitudinal edges of the foil in such a manner that both of its surfaces can be washed over by the liquid.
Such a device is known, for instance, from West German OS 28 49 143.
In liquid containers, the problem frequently arises that the distance of the upper part of the container from the bottom of the container varies considerably since the manufacturing tolerances are large and since, due to the weight of the liquid contained in it, the bottom of the container bulges out to a greater or lesser extent depending on the level to which it is filled. In order, nevertheless, to obtain a sufficiently high accuracy of measurement even in the region of lower levels of liquid in the container, elastic foil holders have been developed which rest resiliently against the bottom of the container and can thus follow along the movements of the bottom of the container. It is also known to hold a rigid foil holder continuously against the bottom of the container by means of a spring. Such constructions are, however, rather expensive and are seldom used because of cost considerations.
The object of the present invention is to develop a device for electrically measuring a liquid level in which the protective tube which carries the conductive foil is capable of contacting the bottom of the container regardless of container tolerances or changes in container shape without the necessity of special hold-down devices or a complicated development of the measuring device.